AGONIST-INDUCED DESENSITIZATION OF A(2B) ADENOSINE RECEPTORS

Agonist-induced desensitization has been described for the A(1), A(2A) , and A(3) adenosine receptor subtypes of the G protein-coupled recept or superfamily. Desensitization of the fourth adenosine receptor subty pe, the A(2B) adenosine receptor (A(2B)R), has not been studied extens ively. We sought to determine whether the A(2B)R is subject to agonist -induced desensitization. COS 7 cells, which exhibit endogenous expres sion of the A(2B)R, and transfected CHO cells, which stably express a modified rat A(2B)R bearing a 5' FLAG epitope tag, were studied. Cycli c AMP (cAMP) responsiveness to an acute challenge was measured after p retreating (desensitizing) cells with the adenosine receptor agonist 5 '-N-ethylcarboxamidoadenosine (NECA). Incubation with NECA resulted in hyporesponsiveness to acute agonist challenge in both COS 7 and trans fected CHO cells. Desensitized cells exhibited restoration of cAMP res ponses after recovery for 24 hr in growth medium. Choleratoxin-induced cAMP responses were preserved in desensitized cells, and high concent rations of NECA were unable to overcome the desensitization. Membrane levels of the epitope-tagged A(2B)R were assessed by western blot in t ransiently transfected COS 7 cells. The expression of epitope-tagged A (2B)Rs was not different between control and desensitized cells. In no rthern blot analysis, levels of endogenous A(2B)R mRNA were similar in control and desensitized COS 7 cells. We conclude that the A,,R is su bject to agonist-induced desensitization with preserved expression of A(2B)R mRNA and protein. Uncoupling of the A(2B) adenosine receptor fr om the G protein complex may contribute to the mechanism of desensitiz ation. BIOCHEM PHARMACOL 55;6:813-882, 1998. (C) 1998 Elsevier Science Inc.